The ABO3 compound is a general name of a compound having a crystal structure similar to that of calcium titanate ore (i.e., perovskite). The compound, when it is molded and then sintered, provides a ceramic having excellent dielectricity, piezoelectricity, and semiconductivity (which ceramic is referred to as a dielectric ceramic hereinafter). The sintered body is in wide use today for electric devices such as a communication device and a computer as a capacitor, a filter, an ignition element, a thermistor, or the like.
In recent years, an electric device has been more miniaturized and improved in performance. Accordingly, parts of an electric device have been also required to be miniaturized and improved in performance, and thus processes for production of dielectric ceramic, e.g., a blending technique, a molding technique, and a sintering technique, among others, have been variously improved. However, such improvement in the processes for production has mostly reached to a limit, and thus it is necessary to improve raw materials in order to obtain a more excellent dielectric ceramic. That is, an ABO3 compound having an average particle diameter of 1 μm or less, preferably 0.5 μm or less, a uniform spherical shape, and excellent dispersibility is required.
The reason why such an ABO3 compound is required is as follows. That is, if the compound has a small particle diameter, it will have increased surface energy. If the compound is spherical and has uniform particle size distribution, it will have increased packing property. Therefore, such an ABO3 compound will have remarkably improved sinterability, so that it will be able to provide a dielectric ceramic densely strengthened by sintering at lower temperature. Furthermore, in order to realize a laminated ceramic capacitor having a thinner and more layered structure, a ceramic green sheet having a thickness of 5 μm or less is required. In this case, an ABO3 compound having an average particle diameter of 1 μm or less, preferably within a range of 0.01 to 0.5 μm, a uniform spherical shape, and excellent dispersibility is also required.
According to “Method and Process for Producing Barium Titanate and Its Composite Particle”, Kyoichi Sasaki, Journal of the Society of Powder Technology (Japan), The Society of Powder Technology (Japan), 1997, Vol. 34, No. 11, pp. 862-874, an ABO3 compound represented by barium titanate has been conventionally produced by a solid phase method comprising steps of mixing barium carbonate and titanium oxide, calcining the mixture at 1000° C. or more, and wet pulverizing the resulting product, filtering, drying, and classifying the same. In such a solid phase method, it is necessary to calcine the mixture for a long time at a high temperature in order to complete solid phase reaction of barium carbonate and titanium oxide. However, if the mixture is calcined for a long time at a high temperature, growth of particles cannot be avoided during the calcination. As a result, it is difficult to control a particle diameter of the resulting barium titanate particle to be 1 μm or less. Further, when the resulting barium titanate particles are provided for various applications, the particles are sintered to form a sintered body and then pulverized. The resulting particles have no uniform size distribution, and also the shape is not proper to be dispersed.
In order to solve the above-described problems, a wet process for production of barium titanate has been proposed. As the wet process, there may be mentioned an alkoxide method, a coprecipitation method, an oxalate method, and a hydrothermal synthesis. However, every method has its own important problems.
For example, U.S. Pat. No. 5,087,437 discloses an alkoxide method in which barium titanate is obtained by mixing barium alkoxide and titanium alkoxide, and hydrolyzing the mixture, or alternatively, reacting titanium alkoxide with barium hydroxide. However, the alkoxides to be used are costly, and the resulting by-produced alcohol needs to be recovered. Thus, the method is not suitable for industrial application.
On the other hand, the coprecipitation method uses a low-priced raw material, and yet the method provides barium titanate powder excellent in sinterability. For example, JP 59-39726A discloses an example of the coprecipitation method in which barium titanate can be obtained by heating and reacting a water-soluble barium salt and a hydrolysis product of titanium compound in the presence of a strong alkali. However, in this method, even when the obtained reaction product is washed, an alkali used in the reaction is hardly removed. Thus, there is a problem that the alkali inevitably gets mixed in the obtained barium titanate powder.
According to the oxalate method, barium titanate can be obtained by reacting titanium tetrachloride, barium chloride and oxalic acid in water to prepare barium titanyl oxalate, and thermally decomposing the oxalate, as described in, for example, U.S. Pat. No. 2,758,911. Since high purity titanium tetrachloride and barium chloride to be used as raw materials are easily available, high purity barium titanate can be easily obtained by the method. However, precipitate of barium titanyl oxalate obtained by the method is composed of large aggregate and the skeleton of the aggregate is remained at the time of calcination. Thus, the method is apt to produce coarse particles. Further, when the obtained barium titanate powder is sintered to form barium titanate ceramic, there is a problem that the resulting ceramic has large dielectric loss.
Further, there has been known a hydrothermal synthesis as a production method of barium titanate in which a mixture of barium hydroxide and a hydroxide or oxide of titanium is subjected to hydrothermal treatment. The method provides fine and uniform barium titanate having particularly excellent dispersibility. Thus, it has been known that barium titanate obtained by the hydrothermal synthesis can be preferably usable for preparing a laminated ceramic capacitor having a thinner and more layered structure.
For example, U.S. Pat. No. 2,193,563 and “Wet Synthesis of Barium Titanate (BaTiO3)”, Kiichiro Kubo, “Journal of the Chemical Society of Japan”, 1968, Vol. 71, No. 1, pp. 114-118, describe hydrothermal synthesis to produce barium titanate.
However, in a hydrothermal synthesis, since the reaction of barium hydroxide and a hydroxide or oxide of titanium does not proceed to completion, unreacted titanium components get mixed as solid with barium titanate obtained by the reaction while unreacted barium hydroxide remains dissolved in the reaction mixture. Therefore, when the reaction mixture is filtered and washed with water to separate the obtained barium titanate as solid from the reaction mixture, the water-soluble barium components are removed from the obtained barium titanate. Thus, the obtained barium titanate excessively contains titanium components. Therefore, even if such barium titanate powder is sintered, only a sintered body excessively containing titanium components is obtained. Further, when barium titanate is produced by hydrothermal synthesis, reaction rates of raw materials are slightly varied for every reaction. Thus, since the reaction rates cannot be strictly controlled so that the resulting barium titanate has a predetermined Ba/Ti ratio for use as an electric material, the barium titanate produced by hydrothermal synthesis is not suitable for use as a material of a dielectric ceramic.
In order to solve the above-described problems involved in hydrothermal synthesis of an ABO3 compound, JP 61-31345A has proposed a method in which an A group element dissolved in an aqueous medium is made to be insoluble after hydrothermal reaction to control an A/B ratio of ABO3 compound obtained. This method has been practically used as a method from production of material for a dielectric ceramic.
The problem how to control an A/B ratio in a production of an ABO3 compound by hydrothermal synthesis has been solved in this way. However, barium titanate produced by hydrothermal synthesis has a new problem that hydroxyl groups are included in oxygen lattices of particles. That is, for example, according to “Defective Chemistry and Minute Structure of Barium Titanate by Hydrothermal Synthesis”, D. F. K Henning et al., J. Am. Ceram. Soc., 2001, Vol. 84, No. 1, pp. 179-182, when such barium titanate is heated at a temperature of 100 to 600° C., dehydration reaction takes place to produce pores having a nano meter (nm) size in particles. When such barium titanate is formed to a thin layered sintered body, the pores in the barium titanate particles cause crack or delamination, thereby preventing a laminated ceramic capacitor to have a thinner and more layered structure. Therefore, this problem has been required to be solved.
As mentioned above, there is a problem that a conventional ABO3 compound represented by barium titanate cannot fully meets the requirements to miniaturize electric parts of a capacitor, a filter, a thermistor, and the like, and to improve the performance of those. Further, in the hydrothermal synthesis which has been known as an excellent method for production to provide uniform and fine spherical particles having excellent dispersibility, pores having a nano meter size are formed in particles by dehydration at the time of heating. When barium titanate having pores in particles is used to produce a laminated ceramic capacitor, crack or delamination is undesirably caused.
The invention has been completed to solve the above-described problems in the production of an ABO3 compound represented by barium titanate. Therefore, it is an object of the invention to provide a process for production of a composition comprising an ABO3 compound by an improved hydrothermal synthesis, wherein the composition obtained is in the form of uniform spherical particles which have an average particle diameter of 1 μm or less, preferably within a range of 0.01 to 0.5 μm, high crystallinity, and a controlled A/B ratio as desired, as well as few internal pores having a nano meter (nm) size in the crystalline particles. The ABO3 compound having few pores in the crystalline particles can be advantageously used to produce a laminated ceramic capacitor.